Polarized and multiplexed optical signals are increasingly used for large-capacity optical communications and importance of polarization beam splitters for coupling and splitting polarized waves is increasing. Particularly, a waveguide-type polarization beam splitter is attracting attention because the waveguide-type polarization beam splitter can be integrated with other waveguide-type devices such as a coupler, a delayed interferometer, and an optical hybrid. The waveguide-type polarization beam splitter generally achieves a polarization wave coupling-splitting function as follows. A phase difference of π is provided between a TE polarization and a TM polarization in a configuration of a Mach-Zehnder interferometer (MZI) and the phase difference of the TE polarization in the interferometer is set to 0 (or π) while the phase difference of the TM polarization in the interferometer is set to π (or 0).
FIG. 1 shows an example of a conventional waveguide-type polarization beam splitter. The conventional waveguide-type polarization beam splitter includes input optical waveguides 101a, 101b, a first optical coupler 102, a pair of waveguide arms 103, a groove 104 provided to extend across the waveguide arms, quarter wave plates 105a, 105b of angles of 0° and 90° which are inserted in the groove 104, a second optical coupler 106, and output optical waveguides 107a, 107b (see Patent Literature 1). Since the wave plates inserted in the respective arms give the phase difference between the polarized waves in this method, a polarization beam splitter excellent in temperature characteristics can be achieved.